High temperature ferromagnetic cobalt-base alloy



United States Patent 3,415,643 HIGH TEMPERATURE FERROMAGNETICCOBALT-BASE ALLOY John C. Freche, Fairview Park, Richard L. Ashbrook,Berea, Gary D. Sandrock, Cleveland, and Robert Lewis Dreshfield, ParmaHeights, Ohio, assignors t0 the United States of America as representedby the Administrator of the National Aeronautics and SpaceAdministration N0 Drawing. Filed May 31, 1966, Ser. No. 554,950

9 Claims. (Cl. 75-170) The invention described herein was made byemployees of the United States Government and may be manufactured andused by or for the Government for governmental purposes without thepayment of any royalties thereon or therefor.

The present invention relates to improved alloys having high strength attemperatures up to 1500 F. while exhibiting high magnetic induction atlow applied fields. The invention is particularly concerned withproviding improved materials for use in electrical power generatingequipment that is subjected to the environment of liquid metal vapor orto the vacuum of space.

Present day aerospace vehicles require space power systems having highefiiciency. In order to increase the chiciency of a space power system,it is necessary to decrease the amount of power expended on coolinggenerators and motors. This can be accomplished by using rotor materialsthat have high Curie temperatures, high magnetic induction for lowapplied fields, and high strength at high temperatures therebyaccommodating high operating temperatures.

Commercially available materials for space power system rotors includesteels, such as AISI 4340, and high strength tool steel, such as H-ll.Each of these steels has certain limitations. While at low temperatures,these materials have high magnetic induction for low applied fields.However, the strength of these steels falls off rapidly at about 1000 F.

One of the most desirable structural magnetic materials for use up toll00 to 1150 F. is a cobalt-base alloy called Nivco. However, thismaterial rapidly loses strength above 1200 F., and its magneticinduction falls 01f rapidly above 1300 F.

These problems have been solved by the present invention which providesa cobalt-base alloy for use as a material for rotors of electricalgenerators or motors to be operated at temperatures up to axxproximately1400 F. These alloys can be either used as solid rotors or rolled andfabricated into laminated rotors.

It is, therefore, an object of the present invention to provide aferromagnetic cobalt-base alloy having high strength at the elevatedtemperatures of liquid metal vapor in Rankine cycle turbinealternator-s.

Another object of the invention is to provide improved alloy materialshaving high Curie temperatures, high magnetic induction for low appliedfields, and high strength at high temperatures, thereby permitting highoperating temperatures for use in space power systems.

A still further object of the invention is to provide improved alloymaterials for use in aerospace applications which are subjected to theenvironment of liquid metal vapor or to the hard vacuum of space.

These and other objects and advantages of the inven- Patented Dec. 10,1968 tion will be apparent from the specification which follows. Thepresent invention is embodied in alloys having the following nominalcomposition range:

A preferred embodiment of the invention includes a1- loys having thefollowing nominal composition range:

Cobalt From 82.9 percent to 87.9 Tungsten From 7.5 percent to 10Titanium 1 Zirconium 0.5 Carbon 0.6 Iron From 2.5 percent to 5 An alloycomposition used where high strength is of paramount importance has thefollowing nominal composition:

Percent Cobalt 85.9 Tungsten 12.0 Titanium 1.0 Zirconium 0.5 Carbon 0.6

The subject alloys were prepared by vacuum induction melting. However,it is contemplated that these alloys can also be prepared by inductionmelting under a protective atmosphere of argon.

The subject alloys were prepared in a vacuum induction melting furnacecapable of reaching pressures as low as 10* torr. and holding pressuresas low as 5 X10 torr. during active boiling of the melt. Raw materialswere charged both into a zirconia crucible as well as into anadditions-maker. After the crucible charge was melted, the melt wasrefined by permitting the oxygen in the charge to react with carbon toproduce a so-called carbon boil. After refining, the portion of thecharge containing the reactive elements, titanium and zirconium, wasadded to the melt which was then heated to an appropriate castingtemperature. The melt was then poured into ceramic shell moldsheated to1600" F.

Although vacuum induction melting was used in the development of thisalloy series, similar alloys have been successfully induction meltedunder an argon atmosphere. These alloys are likewise amenable to meltingby the electro-slag process where protection from the atmosphere ispartially provided by a layer of slag rather than solely by vacuum or aninert gas.

The above listed alloys derive their high elevated temperature strengthfrom the solid solution strengthening of cobalt by tungsten, by theprecipitation of an intermediate phase of WCo and by the presence ofdispersed tungsten, titanium and zirconium carbides. The good magneticand strength properties are the result of a compromise between thetungsten and iron content in the alloy. The iron contributes to the highinduction and structural stability while the tungsten contributes to thehigh strength.

Various magnetic and mechanical properties of the preferred compositioncompared with those of one of the applied fields at elevatedtemperatures consisting essentially of from 5% to 14% tungsten, from .3%to 1.8%

TABLE l.MAGNETIC AND MECHANICAL PROPERTIES OF SELECTED ALLOYS Testtempera- Alloy Property ture, F

7.5W-2.5Fe W-5I e 12 W Nivco Ultimate tensile strength, p.s.i 1, 300 57,200 58, 000 59, 000 3 95, 000 1, 400 46, 350 48, 600 52, 600 49, 500 l,500 41, 000 49, 000 50, 000 3 29, 000 1, 600 22, 000 1, 700 33, 600 32,200 35, 500 Stress rupture life at stress 0145,000 p.s.i. in hours 1,2002 50 90 1 1, 268 750 300 25 433 1, 290 30 1, 400 1. 2 96 392 1 Magneticinduction in kilogausses for field of 100 oersteds a 1, 400 10. 3 9. 28. 2 9. 4 Curie temperature, F 2 1,820 1, 710 1, 600 1, 600

1 Stress 50,000 p.s.i. Extrapolated. 3 Interpolated.

There are other alloys suitable for magnetic applications at elevatedtemperatures. Among these are H-11 and 18% nickel-maraging steel.However, their use is limited to temperatures below 1000 F. because ofmicrostructural instabilities. The lowest test temperature shown inTable I is 1200" F.

It has also been found that the stress rupture properties of the alloyseries can be improved by heat treating. Two such heat treatments havebeen found to improve the stress rupture properties of the alloyCo-7.5W-2.5Fe-1Ti- O.5Zr-O.6C in the range from l200 to 1400 F.According to the invention, the alloy was heated for 72 hours at either1700 F. or 1500 F. Table II illustrates the effect of such heattreatments on mechanical properties of the alloy as compared to Nivco10.

titanium, from 0.1% to 1% zirconium, from 0.3% to 0.7% carbon, up to 10%iron, and the rest essentially cobalt.

2. A cobalt base alloy having a high Curie temperature, high strengthand high magnetic induction for low applied fields at elevatedtemperatures consisting essentially of 1.0% titanium, 0.5% zirconium,0.6% carbon, from 7.5% to 10% tungsten, 2.5% to 5% iron, and the restcobalt.

3. A cobalt base alloy as claimed in claim 2 containing 7.5% tungsten,2.5% iron and about 87.9% cobalt.

4. A cobalt base alloy as claimed in claim 2 including 10% tungsten, 5%iron, and 82.9% cobalt.

5. A- cobalt base alloy as claimed in claim 2 including 10% tungsten,2.5% iron, and about 85.4% cobalt.

TABLE II.EFFECT OF HEAT TREATMENT ON MECHANICAL PROPERTIES OF SELECTEDALLOY Alloy Property Test temperature F. Heat-treated-aging N ivcoAs-east temperature 1,700 F. 1,500 E. Average ultimate tensile strength(p.s.i.) 1,200 55, 800 53, 400 53, 300

1, 300 57, 200 51, 500 51, 100 95, 000 1, 400 46, 350 46, 000 46, 40049, 500 1, 500 41, 100 42, 300 29, 000 Average stress repture life at45,000 p.s.i. in hours. 1, 200 1 2, 500 750 1, 300 400 l, 700 Averagestress rupture life at 40,000 p.s.i. in hours. 1, 400 25 30 300 0. 4Magnetic induction in kilogausses 1, 400 10. 3 10. 3 9. 4

1 Extrapolated.

The beneficial technical effect of heat treatment on stress ruptureproperties is associated with the formation of a finely dispersedprecipitate in the material. The aging treatments do not significantlyaffect the tensile strength. Depending on the test conditions, bothslight increases and slight decreases in tensile strength have beenobserved. However, the greatest room temperature ductility was obtainedafter aging at 1700 F. for 72 hours. Both heat treatments resulted inimproved properties over the as cast condition and over the commercialalloy Nivco 10.

The examples set forth above describe the properties of castings made inaccordance with the invention. It is contemplated that the cast alloyswill be worked into sheet material by forging or rolling for use inmaking laminated structures, such as rotors, when it is desired tominimize eddy current losses. Conventional hot rolling techniques havebeen used to obtain sheet material for these alloys. A satisfactoryannealing treatment for the sheet has been found to be one-half hour at2350 F.

It is understood that equivalents or modifications of or substitutionsfor parts of the above-described embodiments of the invention may bemade without departing from the spirit of the invention or the scope ofthe subjoined claims.

What is claimed is:

1. A cobalt base alloy having a high Curie temperature, high strengthand high magnetic induction for low 6. A cobalt base alloy consistingessentially of 12% tungsten, 1.0% titanium, 0.5% zirconium, 0.6% carbon,and about 85.9% cobalt.

7. A method of making a high strength ferromagnetic material havingimproved electrical properties at temperatures upto 1400 F. comprisingthe steps of casting an alloy having a composition essentially in therange of about 5% to 14% tungsten, .3% to 1.8% titanium, 0.1% to 1%zirconium, 0.3% to 0.7% carbon, up to 10% iron, and the rest cobalt, and

heat treating the alloy to improve the strength thereof by aging forabout 72 hours at a temperature in the range from about 1300 F. to about1700 F.

8. A method of making a high strength ferromagnetic material as claimedin claim 7 including the step of working the cast alloy into sheet.

9. A method of making a high strength ferromagnetic material as claimedin claim 8 including the step of annealing the. sheet at 2350 F. forone-half hour.

References Cited UNITED STATES PATENTS 9/1966 Freche et al. 75l7010/1966 Freche et al. 75-17O

1. A COBALT BASE ALLOY HAVING A HIGH CURIE TEMPERATURE, HIGH STRENGTHAND HIGH MAGNETIC INDUCTION FOR LOW APPLIED FIELDS AT ELEVATEDTEMPERATURES CONSISTING ESSENTIALLY OF FROM 5% TO 14% TUNGSTEN, FROM .3%TO 1.8% TITANIUM, FROM 0.1% TO 1% ZIRCONIUM, FROM 0.3% TO 0.7% CARBON,UP TO 10% IRON, AND THE REST ESSENTIALLY COBALT.